Method for providing a heat treated filled and closed metal can

ABSTRACT

Method for providing a heat treated filled and closed can, including the consecutive steps of: filling a metal cup, closing the metal cup with a lid making a gas tight heat treatable can, heat treating the can, wherein measures are taken to achieve an under-pressure in the can after closing the cup and wherein the can is chosen that is either of a flexible type on the one hand or of a rigid type with increased strength provided with a seal on lid on the other hand. The measures comprise a step belonging to the group of steps consisting of: using a partly frozen filling; having the filling include constituents that interact after closing so as to lower the specific volume of the filling in the can; adding steam to the cup after filling and before closing; closing the cup under sub-atmospheric pressure; and partly evacuating the can after closing.

The invention relates to a method for providing a heat treated filledand closed metal can.

A heat treated filled and closed metal can will usually contain a foodfor humans or animals, which food is or is to be heat-treated in theclosed metal packaging after filling. The heat treatment the foodundergoes is in principle related to the type of food, and may furthervary per recipe and producer.

The metal of the can is usually steel or aluminium. In the steel andaluminium industry as well as in the packaging industry and in the foodindustry it is continuously sought to improve the packaging e.g.regarding the amount of material consumed when producing a can, or theamount of material which can be recycled or the appearance of a can tothe consumer.

An example of the achievements of continuous innovation is a canaccording to the Le Carré® concept, which is a multi-panel type of canhaving flat shell parts, as disclosed in e.g. EP 1005428 entitled “metalbody for packaging purposes, for example a food can”.

According to EP 1005428, by providing a flexible can it is possible towork a method for heat treating, for example sterilising a filled can inan autoclave, whereby the can needs to be handled far less critically interms of pressure. In practice this means that the pressure control ofthe autoclave is far easier to achieve. As long as the pressure in theautoclave is higher than the pressure in the can nothing can go wrong.

Although the concept of Le Carré® as set out above was very promisingthere is a problem that it is not always economically attractive toperform heat treatment in autoclaves albeit under flexible conditions.Commercial sterilisation autoclaves operate batch-wise and batchprocesses are not economically attractive for all food stuffs in thefood packagings under consideration.

Further, there is a need to find better solutions for accessibility ofcanned food stuff by providing more easily opening closures and it isknown that such closures because of their easy opening characteristicswill be more vulnerable to—even quite low—internal over-pressuresituations, especially when such an over-pressure situation is combinedwith high temperature, like a sterilisation temperature of 120° C. ormore, and time, e.g. during a sterilisation period of half an hour ormore. In the present document, the term over-pressure denotes a pressurein the closed can that is higher than the pressure outside the closedcan. Similarly the term under-pressure denotes a pressure in the closedcan that is lower than the pressure outside the closed can.

This problem is now overcome or reduced substantially by the firstembodiment of the invention defined as a method for providing a heattreated filled and closed can, comprising the consecutive steps of:

-   -   filling a metal cup,    -   closing the metal cup with a lid making a gas tight heat        treatable can,    -   heat treating the can,        wherein measures are taken to achieve an under-pressure in the        can after closing the cup characterised in that the can is of a        flexible type.

The measures comprise a step belonging to the group of steps consistingof:

-   -   using a partly frozen filling;    -   having the filling comprise constituents that interact after        closing so as to lower the specific volume of the filling in the        can;    -   adding steam to the cup after filling and before closing;    -   closing the cup under sub-atmospheric pressure;    -   partly evacuating the can after closing;

The term flexible denotes that the volume the closed and filled canoccupies increases substantially if there is only a slight over-pressurein the can and decreases substantially if there is only a slightunder-pressure in the can.

By choosing in this method a can with this feature of flexibility, byshifting from a pressure orientated approach to a volume orientatedapproach advantages are achievable as will elucidated further hereafter.

In this context the filling comprise constituents that interact afterclosing so as to lower the specific volume of the filling in the canmeans for example having the filling comprise constituents that afterthe cup is closed react so as to form a reaction product that occupies alower volume than that of the original constituents, and thisindependently of the effect temperature has on volume.

In an embodiment of the method according to the invention wherein a canis chosen that is of a flexible type, the can is closed with a lid ofthe easy pull off seal on type adhered by a sealant to the metal cup.According to the invention it is now possible to use such a very easilyopenable but over-pressure sensitive lid in spite of the heat treatmentthat would by industry prejudice necessarily cause over-pressure whichwould lead to failure of such over-pressure sensitive seal on type lid.

In preferred embodiments a can is chosen that is of a flexible type thathas a flexibility of more than or equal to 25, preferably 35, theflexibility being quantitatively defined in detail hereafter. Bychoosing a can that has a flexibility of a considerably higher valuethan conventional heat treatable cans, the risk of too high anover-pressure as well as of too high an under-pressure is considerablyreduced.

In a preferred embodiment a can is chosen that is of a flexible typecapable of surviving a volume reduction of more than 7.5%, preferablymore than 10% or even 15% without collapsing. By choosing such a can therisk of collapse in an extreme under-pressure situation is minimised.

The invention is also embodied in a method according to claim 1, whereina cup is chosen that comprises an essentially flat wall panel. Such acup is flexible because of the mechanical properties inherent in anessentially flat panel forming part of a body, in this case the cup.

The aforementioned problem is also overcome or reduced substantially bythe second embodiment of the invention defined as a method for providinga heat treated filled and closed can, comprising the consecutive stepsof:

-   -   filling a metal cup,    -   closing the metal cup with a lid making a gas tight heat        treatable can,    -   heat treating the can,        wherein measures are taken to achieve an under-pressure in the        can after closing the cup characterised in that the can is of a        rigid type and that the can comprises a lid of the easy pull off        type adhered to the metal cup, the measures being of the kind        mentioned above.

The term rigid denotes that the volume the closed and filled canoccupies does not change substantially if there is even a substantialover-pressure in the can and vice versa.

By choosing in this method a can with this feature of rigidity, byshifting in a pressure orientated approach the internal under-pressureto a higher absolute values, thus lowering the maximum internalover-pressure, it is now possible to use a “seal on” can lid, providedthat the rigid can is made strong enough to bear the increased internalunder-pressure as will elucidated further hereafter.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 is a graph of ΔP-ΔV with homogeneous temperature T, and shows theperformance of a Le Carré® fitted with a lid of the easy pull off type(EPOL) and a reference can during sterilisation with varying degrees ofvacuum filling achieved by adding steam to the cup of the can beforeclosing. The line denoted ‘1’ represents Le Carré®, the line denoted ‘2’represents the reference can, the line denoted ‘3’ represents the upperboundaries and the line denoted ‘4’ represents the lower boundariesexplained hereinafter.

The vertical axis denotes the volume change ΔV in ml of the can and thehorizontal axis denotes the pressure difference ΔP over the can in bar.The ΔP-ΔV measurements are performed by pumping a fluidum, in this casewater, into an already filled can (over-pressure situation) or pumpingwater out of a filled can (under-pressure situation). The pressure andvolume changes are measured as the water is pumped in or out.

In FIG. 1 the flexibility line denoted “Le Carré®” (wall 0.13 mm, bottom0.17 mm, EPOL 0.17 mm) extends from the lower left quadrant into theupper right quadrant and the gradient of the line represents theflexibility of the Le Carré® can, of which can a photographicrepresentation is shown in FIG. 2. The flexibility line denoted“reference can” also extends from the lower left quadrant into the upperright quadrant and the gradient of the line represents the flexibilityof a round reference can (diameter 73 mm, 0.14 mm 3 piece steel can withconventional 0.196 mm ends, maximum contents 414 ml at ambientconditions). Defining flexibility as the gradient$\frac{\Delta\quad V}{\Delta\quad P}$of the flexibility line in the interval between ΔV=−10 ml and ΔV=10 mlthe Le Carré® can has a flexibility of approximately 154 which is aboutnine times greater than the reference can which has a flexibility ofapproximately 17.

For completeness it is remarked that to be able to compare theflexibility of different containers, in case a container with adifferent content would have to be tested, for example a container witha maximum content of 500 ml at ambient conditions, the interval to beused to calculate the flexibility will be 500/414*10 which is approx. 12ml.

The upper and lower boundaries represent the extreme process conditionsthe can may be subjected to during the sterilisation process. The upperboundaries are based on a sterilisation process with a sterilisationtemperature of 121° C. and a counter pressure of 2 bar, the lowerboundaries are based on conditions where the temperature is 20° C. andthe counter pressure is also 2 bar.

The specific boundary conditions illustrated in FIG. 1 apply to placinga filling at a temperature of 60° C. in an cylindrical test vessel witha content of also 414 ml, leaving 5% headspace above the filling andapplying pressure to the contents of the test vessel (i.e. to both thefilling and contents of the headspace) via a piston. The externalpressure applied to the test vessel was 2 bar. The pressure and volumechanges across the test vessel were measured with the system at a lowertemperature of 20° C. and an upper temperature of 121° C.

The “normal” boundary represents the situation where there is no steamsupplied to the can cup before closing. However, as the temperature ofthe filling is 60° C., approximately 20% of the air in the headspacewill be replaced. The remaining upper and lower boundary conditions aremarked with percentages that indicate the percentage of air deliberatelyreplaced by adding steam into the headspace. Such partial or wholevacuum filling results in the can having to withstand less over-pressureduring sterilisation.

According to the invention, the over-pressure may be reduced by addingsteam to the cup after filling and before closing but the same effectcan according to the invention also be obtained by using a partly frozenfilling, having the filling comprise constituents that interact afterclosing so as to lower the specific volume of the filling in the can,closing the cup under sub-atmospheric pressure and partly evacuating thecan after closing.

From FIG. 1 it can clearly be seen that replacing e.g. 50% of the air inthe headspace with steam reduces the over-pressure in the hot state aswell as increases the under-pressure in the cold state.

Reducing, or even completely avoiding, over-pressure in the can duringthe sterilisation process enables the can to be sealed with a seal onlid, e.g. an easy pull off lid (EPOL), e.g. an EPOL made of ultra-thinpolymer coated packaging steel, without risking failure of the lid. Alid such as an EPOL can be particularly sensitive to over-pressure andthus may give a risk of failing during a conventional sterilisationprocedure. One method of improving the survival rate of cans fitted withEPOLs is applying a carefully controlled sufficient external counterpressure during the sterilisation process to reduce or compensate theover-pressure experienced by the can.

The present invention achieves the same result without requiring theapplication of such counter pressure. In conventional continuoussterilisation processes, e.g. the hydrostatic process, the way ofsupplying additional counter pressure is by adding more stages to theinstallation, which is complicated and expensive. The method of thefirst embodiment of the present invention thus makes it possible tosterilise large amounts of flexible type cans having a flexibility ofe.g. more than 25 in a continuous hydrostatic sterilisation processwithout requiring expensive additional stages to be included in theinstallation. The method of the second embodiment of the presentinvention thus makes it possible to sterilise large amounts of rigidcans having increased strength and a flexibility of e.g. less than 20and fitted with an easy pull off lid in a continuous hydrostaticsterilisation process without requiring expensive additional stages tobe included in the installation.

As can be seen from the lower process boundaries, the lowering of theover-pressure in the can however, also increases the under-pressure inthe can. The “normal” boundary line shows less severe under-pressureconditions than those obtained when 50-100% of the air in the headspaceis replaced. To overcome this the method of the first embodiment of thepresent invention uses a flexible can, able to withstand the increasedunder-pressure.

The method of the second embodiment uses a can of a rigid type withincreased strength fitted with an EPOL lid.

It is remarked that a steam filled rigid round can (diameter approx. 85mm, height approx. 85 mm, made from aluminium thickness 0.24 mm) fittedwith a conventional seamed full aperture easy open lid is known. Such acan is in the market for packaging of e.g. sweet corn. The heat treatedfilled and closed can according to the method of the second embodimentof the present invention however is a rigid can fitted with an easy pulloff seal on lid rather than a conventional full aperture easy open lid.Thus, contrary to current industry expectation, by using the method ofthe present invention it is now possible to apply EPOLs in such cans andto process such cans in straightforward and large scale heat treatmentprocesses without increased risk of failure.

From FIG. 1 it can be seen that the Le Carré® can flexibility linecrosses through and extends beyond the lower boundaries of the processconditions. The Le Carré® can provided according to the method of theinvention will thus not fail even under the most extreme conceivableconditions. The reference can flexibility line however, does not extendto or cross all the lower boundary lines. The reference can is notstrong and rigid enough or flexible enough to withstand extremeunder-pressure and fails.

The method of embodiment one of the present invention using a flexiblecan thus enables such cans to be sterilised without counter pressureeven when fitted with over-pressure sensitive lids such as EPOLs.

Experiments have shown that the flexible Le Carré® can is best able towithstand under-pressure if the headspace is relatively small, e.g. lessthan 8%.

It is remarked that the rigid can is best able to withstandunder-pressure if the headspace is relatively large, e.g. more than 5%.

Although the method has been described in detail with reference to LeCarré® it is clear that the method could be used successfully for otherheat treated, filled and closed flexible cans or idem rigid cans ofincreased strength fitted with lids vulnerable to over-pressure.

1. Method for providing a heat treated filled and closed can, comprisingthe consecutive steps of: filling a metal cup, closing the metal cupwith a lid making a gas tight heat treatable can, heat treating the can,wherein measures are taken to achieve an under-pressure in the can afterclosing the cup, wherein the can is of a flexible type.
 2. Methodaccording to claim 1, wherein the can is closed with a lid of the easypull off seal on type adhered by a sealant to the metal cup.
 3. Methodaccording to claim 1, wherein the can has a flexibility of more than orequal to
 25. 4. Method according to claim 1, wherein the can has aflexibility of more than or equal to
 35. 5. Method according to claim 1,wherein the can is of a flexible type capable of surviving a volumereduction of more than 7.5%, without collapsing.
 6. Method according toclaim 1, wherein the cup comprises an substantially flat wall panel. 7.Method for providing a heat treated filled and closed can, comprisingthe consecutive steps of: filling a metal cup, closing the metal cupwith a lid making a gas tight heat treatable can, heat treating the can,wherein measures are taken to achieve an under-pressure in the can afterclosing the cup, wherein the can is of a rigid type and the cancomprises a lid of the easy pull off type adhered to the metal cup. 8.Method according to claim 1, wherein the can is of a flexible typecapable of surviving a volume reduction of more than 10% withoutcollapsing.
 9. Method according to claim 1, wherein the can is of aflexible type capable of surviving a volume reduction of more than 15%without collapsing.
 10. Method according to claim 1, wherein themeasures comprise at least one step belonging to the group of stepsconsisting of: using a partly frozen filling; having the filling includeconstituents that interact after closing so as to lower the specificvolume of the filling in the can; adding steam to the cup after fillingand before closing; closing the cup under sub-atmospheric pressure; andpartly evacuating the can after closing.
 11. Method according to claim7, wherein the measures comprise at least one step belonging to thegroup of steps consisting of: using a partly frozen filling; having thefilling include constituents that interact after closing so as to lowerthe specific volume of the filling in the can; adding steam to the cupafter filling and before closing; closing the cup under sub-atmosphericpressure; and partly evacuating the can after closing.